Mountain ash trees can be divided into two main types: European mountain ash (Sorbus aucuparia) and American mountain ash (Sorbus Americana). Both types live up to 250 years and grow relatively quickly, providing several seasons of colour to the garden. European mountain ash trees have a moderate growth rate and a more traditional treelike appearance. The American variety grows slowly and tends to look more like a shrub.
Where to Plant
European mountain ash is hardy in United States Department of Agriculture hardiness zones 3 through 7 (average annual minimum temperatures of minus 40 to 10 degrees Fahrenheit). American mountain ash is hardy in USDA zones 2 through 6 (minus 10.0 to -17.7 degrees C). Both varieties prefer planting locations that receive full sunlight. Mountain ash is adaptable to a variety of soil conditions but prefers moist, well-drained soil -- they do not tolerate drought.
Mountain ash is a deciduous tree that sports colourful fall foliage before it drops its leaves for winter. Leaves on European mountain ash turn to shades of yellow or reddish-purple in autumn. American mountain ash features vibrant yellow or dull red foliage during the fall. During the growing season, trees are adorned with toothed, medium-green leaves that range from 5 to 12 inches, depending on the variety.
American mountain ash trees bloom with clusters of small, white flowers in June. Flowers give way to vibrant red berries that form during mid to late summer and persist into fall. European varieties experience a similar bloom time, but feature larger clusters of flowers. The orange berries are larger than those of American varieties and remain on the tree through winter.
Their compact size and colourful berries entice many landscapers to add mountain ash trees to the landscape or garden. European mountain ash trees grow to heights that range from 20 to 35 feet, with a 15- to 25-foot spread. American mountain ash trees grow to heights of 10 to 30 feet with a similar spread.
Mountain ash is susceptible to sun scald on the tree's trunk, damage from sapsuckers and fire blight, a bacterial disease. Symptoms of fire blight include a water-soaked appearance to the tree's blossoms followed by flower death. Additional symptoms include colour change of young shoots or droplets of oozing material from the tree during hot, humid weather. Fire blight can lend a scorched appearance to the tree and eventually cause fatal damage.